Recently, liquid crystal displays (LCD) have been widely applied in electrical products, due to the rapid progress of optical technology and semiconductor technology. Moreover, with the advantages of high image quality, compact size, light weight, low driving voltage, and low power consumption, LCDs have been introduced into portable computers, personal digital assistants, color televisions, and replaced gradually the cathode ray tubes (CRT) used for conventional displays. LCDs are becoming a mainstream display apparatus.
The main part of an LCD is a liquid crystal (LC) unit composed of two parallel transparent substrates with LC sealed therein. The main trend in LCDs is thin film transistor (TFT) LCD. The fabrication processes of a TFT-LCD can be divided into four parts: TFT array process, color filter (CF) process, LC cell assembly process, liquid crystal module (LCM) process.
The TFT array process is used to fabricate a TFT substrate. Each TFT respectively aligns with one pixel electrode. The CF process is used to fabricate color filter substrate. A color filter layer composed of different color filter sheet is on the color filter substrate, and a black matrix layer surrounds each color filter sheet.
The LC cell assembly process is used to parallel assemble TFT substrate and CF substrate. Bead spacers are spread between the TFT substrate and the CF substrate to maintain a fixed distance, i.e. a cell gap, therebetween. LC is injected into the cell gap and then the injection opening is sealed. Basically, each pixel electrode respectively corresponds to one color filter sheet, and the black matrix layer covers TFTs and metal lines that connect different TFTs.
The LCM process is used to attach a polarizer to a panel, and electrically connect driver IC and panel circuit. Then a reflector and a back-light are assembled on the panel. After a burn-in step, the LCM process is finished.
Generally, the direction of liquid crystal molecule axes, which are controlled by TFT, determines whether each pixel is pervious to light or not. The color of each pixel is determined by the color of color filter sheet. For example, when light passes through a red color filter sheet, a red spot is shown on the panel. Mixing red, green and blue colors can show full-color images.
Since the molecular axes of liquid crystal molecules, which are between pixel electrodes and color filter sheets, have to be controlled precisely, the color filter and the TFT substrate must be aligned precisely. The tolerable error of alignment is only within several micrometers.
Conventional LC injection is a complicated procedure in the TFT-LCD fabrication process. One drop fill (ODF) process can simplify the fabrication processes. In ODF process, use of a thermal hardening seal can reduce the manufacture cost. But a thermal hardening seal needs a long time to fully harden in a heating chamber. Therefore, the unhardened seal may contact the liquid crystal and pollute the liquid crystal, and the substrates may not glue well.